1. Field of the Invention
The present invention relates to a resist material used in a lithography step of a semiconductor device fabrication process and, more particularly, to a chemical amplification type resist material improved in pattern resolution, vertical shape, and dimensional accuracy.
2. Description of the Prior Art
Recently, with increasing integration degree of LSIs, a demand for formation of high-accuracy fine patterns is becoming increasingly acute. Conventionally, the main current of this fine pattern formation technology (lithography) is an ultraviolet exposure technology which is a combination of an exposure apparatus (stepper) using the g-line or i-line of a mercury lamp and a novolak resist material (to be simply referred to as a resist hereinafter). In addition to improving the performance of a stepper (e.g., increasing the NA of a lens and improving the alignment accuracy), it has been attempted to increase the resolution of a novolak resist. However, if the exposure wavelength is shortened (e.g., deep ultraviolet light such as KrF excimer laser light or mercury arc lamp light near 250 nm is used) in order to further improve the resolving power, a rectangular resist shape cannot be obtained from a novolak resist because the light absorption of a resin and a naphthoquinonediazide photosensitive material is large. In addition, at the sensitivity (usually 100 to 200 mJ/cm.sup.2) of a novolak resist the light intensity of a narrow-band KrF laser light source is lower than (1/5 to 1/10 as low as) those of the g- and i-lines. This undesirably prolongs the exposure time.
To overcome this situation, a chemical amplification type resist was proposed by H. Ito and C. G. Wilson in American Chemical Society Symposium Series, 242, p. 11 (1984). A resist of this type uses a catalytic reaction of an acid generated by a photosensitive acid generator, and so the required amount of an acid is small (on the order of .mu..sub.mol /g). Accordingly, a high-sensitivity resist design is possible. Also, the resist shape can be greatly improved by selecting a low-concentration acid generator and a highly transparent resin.
FIG. 1 shows an example of acid catalysis of a positive chemical amplification type resist (a two-component system consisting of a main resin and an acid generator; in many instances various additives are mixed in the resist) made from a polytert-butoxycarbonyloxystyrene resin. FIG. 2 shows an example of acid catalysis of a negative chemical amplification type resist (a three-component system consisting of a main resin, an acid generator, and a crosslinking agent) consisting of PVP (polyvinylphenol resin) and a melamine crosslinking agent. These reactions are described in C. G. Wilson et al., Journal of Electrochemical Society, 133, p. 181 (1986) and J. W. Thackeray et al., Proceeding of SPIE (The Society of Photo-Optical Instrumentation Engineering), 1086, p. 34 (1989). As illustrated in FIG. 1, a positive resist consists of a resin, in which a polyvinylphenol soluble in a developer is protected by a tert-butoxycarbonyl group, and an acid generator, and becomes soluble in an alkaline developer when the protective group is removed by an acid. On the other hand, a negative resist, FIG. 2, consists of three components, a polyvinylphenyl resin, a melamine crosslinking agent, and an acid generator. An exposed portion of this negative resist becomes insoluble because a crosslinking reaction is encouraged by an acid. TABLE 1 shows a list of known representative acid generators.
TABLE 1 __________________________________________________________________________ NAME OF ACID NO. GENERATOR MOLECULAR FORMULA __________________________________________________________________________ 1 BENZENEDIAZONIUM SALT (M IS As, Sb, OR P) ##STR1## 2 DIPHENYLIODONIUM SALT (M IS As, Sb, OR P) ##STR2## 3 TRIPHENYLSULFONIUM SALT (M IS As, Sb, OR P) ##STR3## 4 2,4-DITRI- CHLOROMETHYL- TRIAZINE DERIVATIVE (R IS ALKYL ##STR4## 5 2,6-DINITRO- BENZYLTOSYLATE ##STR5## 6 p-NITROBENZYL- 9,10-DIETHOXY- ANTHRACENE-2- SULFONATE ##STR6## __________________________________________________________________________
The conventional chemical amplification type resists described above greatly improve the sensitivity and the resolving power. As illustrated in FIG. 3A, however, a resin 12 of a positive resist film 10 which is applied on a silicon substrate 1A is randomly oriented and this gives rise to the following problems. In FIG. 3A, 11A as depicted by a white circle represents acid generator which did not optically react and 11B as depicted by a black circle represents acid generator which optically reacted.
(1) Chemical amplification type resists are different from novolak resists in that almost no optical fading occurs and absorption of exposure light is relatively large even after exposure. Accordingly, the exposure light intensity at the bottom of a resist is small, and so little acid is generated at the bottom of the resist film. As illustrated in FIG. 3B, therefore, resist patterns 10A and 10B are apt to become tapered. In particular, the taper angle .theta., is 80.degree. to 85.degree. in positive resists to which a dye is added to reduce the influence of reflection from the underlying substrate to thereby increase absorption of light (the taper angle .theta. is 105.degree. to 110.degree. in negative resists). This problem is more serious on a substrate having a step as shown in FIG. 4A, since the thickness of a resist coating film varies.
(2) The solubility of a resist is determined by the acid diffusion and catalytic reaction during baking after exposure. The acid diffusion is isotropic, and a large diffusion length readily leads to a decrease in dimensional accuracy and resolution. Additionally, the amount of the generated acid changes in accordance with the density of patterns due to an optical proximity effect (the amount of the generated acid is large in a wide exposure region). Accordingly, as illustrated in FIG. 3B, the positive isolated line pattern 10B is thin; the negative isolated space pattern 10A is thick, and the interconnection to be formed is prone to short-circuiting.
(3) Alkali development also is isotropic; as the development proceeds in the direction of thickness the development proceeds in the horizontal direction on the surface portion of a resist. As a result, the resist pattern is tapered as illustrated in FIG. 3B, and the dimensional variation increases.
To solve the above problems, methods of vertically orienting the resin 12 in a resist have been proposed. That is, a method in which a magnetic field is applied to a wafer coated with a resist to thereby cause polarization orientation is proposed in Japanese Unexamined Patent Publication No. 3-66118, and a method in which an electric field is applied during baking after exposure is proposed in Japanese Unexamined Patent Publication No. 3-159114.
In the former method, however, exposure is performed while a uniform magnetic field is being applied. Accordingly, the exposure apparatus is enlarged in size, and it is also necessary to take account of the influence on electronic parts incorporated into the apparatus. Furthermore, the polarizability of a resin with a large molecular weight (the weight-average molecular weight is 10,000 to a few tens of thousands) used in a resist is small, and so it is in many cases difficult to sufficiently, vertically orient the resin even with application of a magnetic field. The developing machine also is complicated.
On the other hand, in the latter method of applying an electric field it is necessary to apply a uniform electric field during baking after exposure. This requires a special baking apparatus. Also, the concentration of the generated acid in the direction of thickness is controlled by the electric field. However, this concentration in the direction of thickness is sensitive to the field intensity, and consequently the resist dimensions are difficult to control. Furthermore, in an actual semiconductor device fabrication process the device structure varies from one microregion to another in a device (because conductive films and insulating films different in thickness form a multilayered structure). Accordingly, even if a constant electric field is externally applied, the concentration of an acid in the direction of thickness of a resist changes due to different electric fields in different microregions. This results in easy variations in pattern dimensions and shape.